Will only set the direction to 'rtl', if the WordPress locale has
the text direction set to 'rtl'.

Fills in the 'directionality' setting, enables the 'directionality'
plugin, and adds the 'ltr' button to 'toolbar1', formerly
'theme_advanced_buttons1' array keys. These keys are then returned
in the $mce_init (TinyMCE settings) array.